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Hiroshi Matsumoto

Abstract
Wefirst present a brief historical review of the development of technology and scientific research
related to the transmission of electrical energy via radio waves. The idea of radio powertransmission was first conceived by Tesla about a century ago. However,the first practical useof radio waves was for transmitting intelligenceand information, and not for transmitting
electrical power per se. At the close of World War II, engineers and scientists re-examined theoriginal Tesla idea of transmitting electric power to a distant place via radio, as high-power
microwave technology became available. These efforts in 1960's resulted in the idea ofthe Solar
PowerSatellite (SPS) which wasproposed by P. Glaser in 1968. The NASA/DOE concept of the
SPS was extensively developed in the late 1 970's. Afterreviewing the history of microwave powertransmission and related theoretical/experimental
studies from the beginning of this century up
to 1980, we will discuss recent research on microwave powertransmission after 1 980. Our focus
will be on related experiments conducted in the 1980's and 1 990's, including those onground-toground microwave energy transmission, ground-to-aircraft power transmission, and rocket-torocket power transmission. The rocket experiment we discuss was conducted to examine a
possible nonlinear resonant interaction of intense microwaves with the ionospheric plasma. The
result ofthe rocket experiment is further studied in detail by particle model computer simulations,and the results are explained in terms of nonlinear plasma effects. Such problems of interaction
between the mi crowavepowerbeamand the ionosphere must be resolved before space-to-groundand space-to-space power transmission can be realisticallydeveloped.

1. Introduction
On the occasion of the 75th Anniversary of URSI, it is appropriate to re-examine the historical
traces of radio utilization
for transmitting electric power without wires to a distant destination.
Today, radio waves are mainly used for transmitting intelligence and information. However,thethreat of the lack of energy resources, especially for electrical energy, is increasing as a result ofProf. Hiroshi Matsurnoto is with theRadio Atmospheric Science Center
Kyoto University, Uji,
Kyoto 61 1, Japan
Tel: + 81 (774) 33 2532
Fax: + 81 (774)'Space and Radio Science"- 75th Anniversary

31 8463

of URSI155

the population explosion and rapid industrialization
over the globe. Therefore, considering thatthe energy problem on our mother planet Earth, and the crisis of the Earth's environment havebecome urgent issues for mankind, weneed to re-examine the use of radio waves for transmissionof clean electrical energy from one place to another, especially from space tothe ground, without
wires.

There exists a good review paper by W. C. Brown [1] on the history up to 1980 of powertransmission by radio waves. Webriefly describe, in Section 2, the historical footprints of radio
power transmission from a century ago to 1980. In the late 1970's, the NASA/DOE sponsoredextensive studies on the Solar Power Satellite (SPS). The NASA/DOE SPS studies program
contained an evaluation ofthe impact of a microwave power beam on the plasma environment of
the ionosphere. Section 3 reviews the theoretical studies on Ohmic heating of the ionosphere, thethermal self focusing instabilities
caused by the SPS microwave power beamand the related
ionospheric heating experiments by ground-based heating facilities. Following these studies, thepresent author conducted a further study of microwave action on the ionospheric plasma, focusing
on the nonlinear resonant scattering of the microwave power beam by magnetized ionosphericplasma. Section 4 presents a theoretical study of the nonlinear resonant interaction of a highpower microwave beam with ionospheric plasma, and a rocket experiment called MINIX(Microwave-Ionosphere Nonlinear Interaction experiment) which was conducted to test thetheoreticalestimate of nonlinear resonant interactions.
Extensive computer simulations of
nonlinear resonant interactions were carried out by the present author and his colleagues tointerpret the MINIX result in terms of nonlinear wave-wave-particle interactions. Section 5
describes the computer simulation and its theoretical interpretation.
In Section 6 we outline two recent microwave-driven airplane experiments: SHARP in Canada
and MILAX in Japan. The recent experiment on microwavepower beamsteering using an active
phased array system developed in Japan is described as well. In Section 7 a brief account of a
recent rocket experiment and recent ground-to-ground power transmission is given. In Section 8
weconclude with a summary and discussion for future plans of research and development on
microwave power transmission (MPT).

2. History of microwave power transmission

before 1980

In, 1864, James Clerk Maxwell [2] predicted
the existence of radio waves by means ofmathematical model. Twenty four years later, in 1 888, bolstered by Maxwell's theory, Heinrich
Hertz [3] first succeeded in showing experimental evidence of radio waves by his spark-gap radiotransmitter. This experiment stimulated Marchese Guglielmo Marconi [4], who first achieved
signal transmission by means of radio waves over 10 m in 1895, and over the Atlantic Ocean in
1 901. It was Reginald Fessenden [5] who first succeeded in transmitting continuous wave(CW)for voice telecommunications[6]. Thus, the road to modern radio telecommunication
wasopened
up around the turn of the century. Modemradio utilization
has been directed into the area of radio
telecommunications
for transmission of "intelligenceand information" over rather weak radio
156

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"Spaceand Radio Science" - 75th Anniversary of URSI

waves. This is one main stream of radio utilization
stemming from the Maxwell-Hertz-MarconiFessenden work. However, another stream of work was directed toward a different radio wave
application.
The second stream of radio utilization
was an effort to transmit electricalenergy byradio to a distant place. These two streams are illustrated
in Figure 1.
The idea of radio power transmission
wasfirst conceived and experimented
onin 1899byNikolaTesla[7,8].He
attempted to distributeten thousand
horse-power under a tension of one
hundred million volts. He said "Thisenergy will be collected all over the
globe preferably
in small amounts,
ranging from a fraction of one to afew horse-power. One of its chiefuses will be the illuminationofisolated homes". He actually built agigantic coil which was connected toa high mast of 2OO-ft with a 3 ftdiameterballat its top (see Figure 2).
He fed 300 kW power to the Tesla
coil resonated at 150 kHz. The RF
potentialat the top sphere reached
100

MV.

Fromthe turning point ofthe century
on, however, radio has been usedmainly for transmitting
intelligenceand information,and very few
attempts have been made to transmit
electrical energy over radio following
Tesla'swork.

The reason for a lack of interest inradio power transmission in the firsthalf of this century is clear. People
were waiting for the invention of a

Fig. I - Historicalfigures
along two stream lines of radio
utilization
around 1900. (A) is a line toward transmission of
intellegenceand information over radio, and (B) is toward
transmitting
electric power over radio"Space and Radio Science"- 75th Anniversaryof URSI157

high-power

microwave device tn

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During World War/I

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l0gy0fthehi^-^^ toDC power until

the 1960's.

158"Space and Radio S,*W'. 75,h Anniversaryof URS,

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The post-war history of research on free-space power transmission is well documented byWilliam C. Brown [1], who was a pioneer of practical microwave power transmission (seereferences in [ 1 ]). It was he [14] who first succeeded in demonstrating a microwave-powered
helicopter in 1964, using 2.45 GHz in the frequency range of2.4 - 2.5 GHz reserved for the ISM(Industrial,
Scientificand Medical) applicationsof radio waves (see Figure 5 in [1]). A power
conversion device from microwave to DC, called a rectenna, was invented [15, 16, 17] and used
for the microwave-powered helicopter.
The first rectenna (shown in Figure 3 in [1]) wascomposed of28 half-wave dipoles terminated in a bridge rectifier using point-contact semiconductor
diodes. Later, the point contact semiconductor diodes were replaced by silicon Schottky-barrier
diodes which raised the microwave-to-DC conversion efficiency from 40 % to 84 %[1], theefficiency being defined as the ratio ofDC output to microwave power absorbed by the rectenna.
The highest record of 84% efficiency was attained in the demonstration of microwave powertransmission in 1975 at the JPL Goldstone Facility [ 1 8]. Power was successfully transferred fromthe transmitting
large parabolic antenna dish to the distant rectenna site over a distance of 1.6 km.
The DC output was 30 kW [18] (see Figure 9 in [10]).
Animportant milestone in the history of microwave power transmission was the three-year study
program called the DOE/NASA Satellite Power System Concept Development and Evaluation
Program, started in 1 977. This program was conducted for the study of the Solar Power Satellite(SPS), which is designed to beam down the electrical powerof5 to 10 GW from one SPS toward
the rectenna site on the ground.

The extensive study of the
SPS

Fig. 4 - Artist concept of Solar Power Satellite. Thepower station willtransmit electric power to the Earth andpossibly to Space Factory,
Space Farmsand Space Cities in addition
160

ended

in

1980,

producinga 670 page
summary document [19].
The concept ofthe SPS was
first proposed by P. E.
Glaser [20] in 1968 to meetboth space-based and earthbased power needs. An
artist's
SPS concept isshown in Figure 4. The SPS
wi l l generate electric powerof the order of several
hundreds to thousands ofmegawatts using photovoltaic cells of sizable area,
and will transmit
thegenerated
power via a
microwave beam to thereceivingrectenna site.
Amongmany technological"Space and Radio Science" - 75th Anniversaryof URSIkey issues which must be overcomebefore the SPS realization, microwave power transmission(MPT) is one of the most important key research issues. The problem contains not only thetechnologicaldevelopment of microwave power transmission with high efficiency and highsafety, but also scientific analysis of microwave impact onto the space plasma environment. We
discuss this in the following three Sections.

3. Review of ohmic heating of the ionospere and thermal self-focusing
instability
by SPS and a related ionospere heating experiment
The SPS studies program, carried out in the latter half of 1970's under the sponsorship of the US
NASA/DOE, contained research on the effects associated with the propagation of intense
microwavebeams through the ionosphere. Twomain effects were pointed out.
The first is the resistive (Ohmic) heating effect due to collisional damping of microwaves. Though
the fraction of wave energy absorbed by the ionospheric plasma is very small, the resultant Ohmic
heating can significantly
modify the local ionospheric thermal balance [21, 22]. The electron
temperature is determined by a balance between heating and cooling processes. The heating by
microwaves is proportional
to square of the electric field of the microwave E2, to square of theratio of the local plasma frequency and the microwave frequency, (fp </)2and to the collision
frequencies vei and venwhere vei and venare the collision frequencies of electrons with ions andneutral particles, respectively. The collision frequency venincreases as the electron temperature
increases. Thus Ohmic heating by intense microwaves can be self-amplifying,
and thereby resultin thermal runaway [22, 23]. Calculation ofthe balance between the enhanced heating and coolinglosses through vibrational excitation of N2 and O2 shows that the electron temperature will beraised several-fold for a microwave power density of23 mW/cm2.It is also found that the electron
density in the E-layer will be increased by 10 - 20 % due to a decrease in the temperaturedependent recombination rate of O2+ and NO2+, while in the D-layer increase in the attachment
rate to O2+ will cause up to a 50 % reduction in the electron number density [21, 23]. As theionospheric heating efficiency varies inversely as the square of the radio frequency, ionosphericheating equivalent to that by the SPS microwave beamcanbe achieved at lower radiated powerby heating at a lower frequency. With this idea, experimental tests of the enhanced electron
heating theory were carried out by the Rice University Group lead by W. E. Gordon [24], using
the 430 MHz radar system at the Arecibo Observatory. A series of underdense (fheater </p) HF
ionospheric modification experiments using the Plattevillehigh power HF (5-10 MHz) heating
facility at Colorado were conducted to simulate the effects of the SPS microwave beam, while
monitoring potential impact upon telecommunication system performance [25, 26]. It was
concluded that there is no significant difference between the telecommunication system performanceof the OMEGAsystem (VLF), the LORAN system (LF) and the AM broadcast system (MF)between the times when the heating facility was operating and when it was not. Thus, the impactof the SPS intense microwave beamon the performance of VLF, LF and MF telecommunication
systems would be minimal [25].

'Space and Radio Science" - 75th Anniversaryof URSI-- 161

The second potential effect ofthe SPS microwave beamonto the ionosphere, studied extensively
in the late 1970's, is the phenomenon of thermal self-focusing [27, 28, 29]. Thermal self-focusing
takes place as a result of a positive feedback loop. Small natural density fluctuationsin theionosphere cause a spatial variation of the refractive index thereby giving rise to a slight focusing
and defocusing of the microwave. This slight inhomogeneous (differential)
heating of theionospheric plasma results in a temperature gradient driving the plasma from the focused region
and thereby amplifying the initial density fluctuations. The self-focusing instability
will eventuallyreach a hydrodynamic equil ibrium creating large-scale ionospheric irregularities.
A self-focusing
experiment was conducted at the Arecibo Observatory using intense HF electromagnetic wave
under the overdense condition (4eater < /p) [30]. The experimental result showed clear selffocusing striations and large-scale structuring of the ionosphere. However, it is noted that theseexperiments wereall conducted under the overdense condition and not the underdense condition.
The two main effects described above are caused basically by the non-resonant heating of theplasma by the intense electric field ofthe SPS microwave. Resonant interactions ofthe microwave
beam with the ionospheric plasma are another interesting research area. Parametric excitation ofionospheric plasma waves was studied for multiple-frequency
electromagnetic radiation. Theinteraction between two high frequency microwaves and a multiple of the ionospheric resonant
frequency (such as the electron plasma frequency) was studied [31 , 32] utilizing amodel ofan upgoing pilot microwave signal operating at a frequency slightly separated from the downcoming
powerbeam, andbeat wavesgenerated within the finite width ofthe main downcomingenergy beam.
The US government suspended the NASA/DOE program study partly due to budget problems andpartly because ofthe apparent recovery from the oil shock in the 1970's, although the NASA/DOE
final report concluded that "no factors that would preclude the SPS research and development arefound in the light of the highly potential future energy crisis".

4. Theory and rocket experiment (MINIX) on nonlinear plasma wave
excitation by microwave power beamin the ionosphere
Most of the experiments conducted in the 1970's on the potential impact of the SPS microwave
onto the ionosphere were conducted by the ground-based heating facility using much lower
frequency than the SPS microwave frequency of 2.45 GHz. Therefore, the realistic resonantinteraction of the SPS microwave with natural resonance frequency bands of the order of several
MHzto several kHz may not have been adequately estimated. The resonant interaction naturally
involves electrostatic
waves which can be detected much moreeasily by in-situ measurement thanby ground-based radar diagnostics. Based on such idea, an in-situ rocket experiment of radiating
an intense 2.45 GHz microwave into the ionospheric plasma was proposed by the present author
in the early 1980's in Japan. The project was named MINIX which stands for Microwave
Ionosphere Nonlinear Interaction experiment.

j52

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---"Spaceand Radio Science" - 75th Anniversary

ofURSIThe plasma response to the injected microwave powerbeamwasmonitored by diagnostic sensors
and receivers onboard the rocket. Preceding the MINIX experiment, Matsumoto [33, 34]
numerically evaluated the growth rate of the resonant instabilities
of electron plasma wavesand
ion acoustic waves as a result of Ramanand Brillouin scattering of the SPS intense microwave
under model ionospheric plasma parameters. The growth rate of Langmuir waves as a result of
the Ramanscattering of the microwave is given [34] by

Ye~

2

+r,

-1/(ri,+r?)zTV+^-/tVnsin^

(1)

UJ

where ne is the electron plasma frequency and Feis the damping rate ofLangmuir waves; the latterincludes two terms resulting from the Landau damping and collisional damping. The quantity F2
represents a collisional
damping of the back-scattered microwave with a frequency ofo)2 =^n2e+c2(k-k0)2

vq =eEo I (me0)o) is the sloshing velocity ofelectrons by the SPS microwave, and (p is an angle
between k and Eo. The suffix o denotes the quantity associated with the SPS microwave. A similar
expression for the growth rate T{ for ion acoustic waves was also obtained [34] (not shown).
Numerical calculation of Fe and Fj versus the ionospheric altitude (see Figure 5) showed that
Langmuir wavesare easily excited by the SPS microwave, while the growing time of ion acoustic
300

600

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Growth rate of electron plasma waves(Rad/sec)
F ig. 5 - Growth rates ofLangmuir waves excited by the Roman scattering ofthe SPS microwave, andfa ion acoustic waves excited by the Brillouin scattering ofthe SPS microwave. The microwavefield
Eq is assumed to be 220 V/m(from [33]).

"Space and Radio Science" - 75th Anniversaryof URSI163

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Fig. 7 - A photograph ofpre-flighttest
scene ofthe MINlXpayload at launchingsite KSC in Japan. The daughter unit withvarious sensors and diagnostic packages
wasseparated by a cranefrom the mothersection. A truncated waveguide antennaused for the side ward transmission isseen in the mother section

Figure 8) to radiate intense microwave
with a frequency of2.45 GHz. The DC

power supply to the magnetrons wasgiven by onboard batteries. A plasma
diagnostic package was installed on
the daughter unit of the rocket. It wascomposed ofa VLF wide band receiver,an HF sweep frequency receiver, ageomagnetic aspect sensor, electron
density and temperature meter, and a
microwavereceiver. Four rod antennas
with a length of2 mwere extended out
from the daughter rocket in the top
plane of the daughter unit to detect
plasma waves which are expected to
be nonlinearly excited by the injected
intense microwave. The HF sweep
frequency recei ver covered a frequency
range from 100 kHz up to
18 MHz with a sweeping
timeof250msec. The VLF
receiver covered a frequency range from 60 Hz
up to 25 kHz. Four paddles
were extended out at thebottom levelof thedaughter unit and were usedforrectennas.
The

Fig. 8 - The truncated wave
guide antenna aperture used
for the forward transmissionof 2.45 GHz microwave
onboard the MINIX payload.

"Space and Radio Science" - 75th Anniversaryof URSI165

Fig. 8 - Payload configuration of
MINIXrocket experiment.Motheranddaughter sections are separated byspring coil during the flight in theion ospherearrangement of these antennas andpaddies is shown in Figure 7.
Figure 9 shows the configurationof the payload instrumentation on
board S-520-6 rocket. The motherunit, which is the section below thelevel of separation
plane inFigure 9, carried the power supply
composed of the DC-battery and
DC-DCconverter, the microwave
transmitter with a time sequencer,
two sets of truncated wave guide
antennas, a Langmuir probe, awide-band telemetry set, a neutral
gas plume of N2 gas, and a TV
monitor camera for monitoring thei n-fl ight separation of the daughter
unit. The neutral gas plume was prepared to create an artificiallysimulated D-layer with highcollision frequency at the height of the lower F layer near the apex of the rocket orbit.

Figure 10 shows the trajectory of the S-520-6 rocket. The MINIX experimental time sequence
along the orbit is also shown in the figure. The thick line parts along the orbit indicate the timeinterval of microwave transmission. The experiment was conducted under three different modes;
The first mode(I) was devoted to the study ofOhmie heating in the ionospheric D-layer and lower
E-layer. In this experimental mode, the microwave
wastransmitted continuously for a long duration of200
240 280
10 seconds. Above the altitude of 100 km, thesecond mode (II) was in operation where themicrowave was transmitted intermittently with a 5
sec transmission followed by a 5 sec pause-oftransmission period for the measurement of theplasma response. Under the mode (II), themicrowavewasradiated radially, i.e., in the direction
perpendicular to the spin axis of the rocket. After
100
t50
200
250
300km
the separation of the daughter unit near the apex ofFig. 10 - The trajectory ofS-520-6 rocket
the orbit around 220 km altitude, the experimental
and the time sequence ofthe MINIX
modewas switched to the mode (III) where thealong the orbit

166"Space and Radio Science" - 75th Anniversaryof URSImicrowave wastransmitted forward in the axial direction from the truncated waveguide antennafacing the leaving daughter unit (see Figure 6), with the same ON-OFFtime sequence as that in
the mode II. The modes (II) and (III) were mainly prepared to detect the theoretically predicted
excitation of plasma waves through nonlinear resonant interaction of the transmitted intense
microwave.
All ofthe instrumentation on board the MINIX rocket worked perfectly and provided useful data.
The measurement of the variation of electron temperature showed no temperature difference
between ONand OFF periods of the microwave transmission during the mode (I). It turned out
later with the use of pre-launch plasma chamber data on plasma heating by MINIX transmitter[39] that the estimated maximumtemperature increase due to the Ohmic heating for the MINIX
situation is below 100 K which is lower than the detectable limit by the Langmuir probe used in
the MINIX. It is also noted that the effective time of microwave exposure to the ionosphericplasma particles was too short compared to the characteristic
time for the Ohmic heating. The
plasma volume illuminated by the intense microwave with a power density which is comparable
to or higher than that ofthe SPS microwave is limited to within the distance of 2 m from the centerof the truncated waveguide antenna. The power density and the electric field intensity of the
microwave radiated by the magnetron plus waveguide antenna system are shown in Figure 1 1.
=

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Fig. 1 1 - Power density and electricfield intensity ofthe radiated microwave as afunction ofdistancefrom the MINIX truncated waveguide antenna. The right panel
shows the antenna pattern of the transmitting antenna.

The sweep frequency analyzer (SFA) in the HF range measured the spectrum of ionosphericplasma waves and broadcasting waves reaching the rocket altitude from the ground. The SFA
detected strong plasma waves at certain frequency bands when and only when the microwave wastransmitted. Theplasma wavespectra from 100 kHz to 18 MHz are shown in Figure 12. The upper"Space and Radio Science" - 75th Anniversaryof URSI167

Fig. 12 - Plasma wavespectra observed by HF
wavereceiver onboard the daughter unit of the
MINIXrocket experiment. The upper spectral line
isforthe time whenthe intense 2. 45 GHzmicrowave
is transmitted into the ionospheric plasma. The
lower spectral line is for the time when the
microwaveis not transmitted. The spectral peaks
above 10 MHz are due to the broadcasting waves
reaching from the ground.
10

15

(MHz)

spectral curve is for the period ofthe microwave
transmission, while the lower is for the notransmission period. The spectra above 10
MHzare not different from each other, but those below 10 MHz show a clear difference. The
shaded part shows the enhancement of the spectral components due to additional excitation of theplasma wavesby the intense microwave. In Figure 1 2, the local electron cyclotron frequency FH
and its harmonics and the local electron plasma frequency are indicated on the horizontal
frequency axis. The error bar in the electron plasma frequency was due to the ambiguity indetermining the number density and temperature from the Langmuir probe. The enhancement in
the spectral intensity is seen in two different characteristicfrequency ranges. One is seen at odd
halfharmonics ofthe local electron cyclotron frequency in the range from 1.5 MHzto 3.5 MHz.
The frequency range of these waves did not change with altitude as the local electron cyclotron
frequency is almost constant in the orbit range of the rocket. Taking into account the observed
frequency range and discrete peaks at odd half cyclotron frequencies, we explained these waves
in terms of electron cyclotron harmonic (ECH) waves.The other enhancement is seen above thelocal electron plasma frequency ranging from 5 MHz to 8 MHz. The frequency range shifts
upward with the altitude of the rocket, and accordingly with the increasing plasma density in the
E-to F-layers ofthe ionosphere. Weexplained the enhancement of this frequency above the local
electron plasma frequency in terms of Langmuir waves due to the excitation by the intense
microwave through Ramanscattering.
Fh

2Fh3FhFREQUENCY

The VLF wide band receiver could not pick up the expected ion acoustic wavethrough Brillouinscattering. This was not surprising because the theoreticallycalculated [34] growth rate of the ion
acoustic waves is not large enough to make these wavesgrow to the observable amplitude withina short time illuminated by the microwave at a fixed point ofthe ionosphere. The illuminated time
wasabout 1 msec, which is calculated by the size ofthe plasma volume illuminated by the intense(E > 200 V/m) microwave, while the theoreticallycalculated growing time is longerthan 10 msec.
The result of the MINIX on nonlinear plasma wave excitation due to intense 2.45 GHz
microwaves wasnot muchdifferent from what had been predicted by the theoretical calculation.
A similar but more complicated expression of the growth rate of the electron cyclotron waves
including upper hybrid waves was obtained based on the nonlinear kinetic theory of resonant
three-wave coupling[40, 41]. It is expressed as

168'Space and Radio Science"- 75th Anniversaryof URSI(3)where £0 *s the intensity
coefficientsdefined byd-f = ifrEoElof the incident

intensemicrowave, and /3j and (32 are the coupling(5)

where Ei and E2 are the electric field of the backscattered microwave and the excited electron
cyclotron wave. With a lengthy calculation and by use ofthe resonant condition (2), we obtain the
final complicated expression of /3j and j32 (eqs. (80) - (93) in [41]) which contains both tensor
elements of the linear dispersion relation and the integral containing resonant terms at higher
harmonics of electron cyclotron frequencies. Numerical calculation of the growth rate shows thatthe electron cyclotron harmonic waves are also excited by the intense microwave underionospheric conditions.

Despite the qualitative agreement with these theoretical predictions, the MINIX result showed thefollowing features which are not consistent with the above theory. The first point is that theobserved spectrum of the excited Langmuir waves are not monochromatic, nor quasimonochromatic, but of broad-band nature in contrast to the monochromaticity predicted by theory
based on the resonance condition (2). The second point is that the intensity of the electron
cyclotron waves is higher than the Langmuir wave intensity. The second point clearly contradicts
the theoretical prediction. The saturation level of the nonlinearly excited plasma waves (bothLangmuir and ECH waves) through the nonlinear resonant three wave-coupling is proportional
to the growth rate divided by the product of the coupling coefficients j80 j^i à" The theory shows tha
the saturation level ofthe Langmuir waves should be larger than that ofthe ECH waves. However,
the amplitude of the Langmuir waves excited by the microwave in the MINIX was smaller thanthat ofthe ECH. Thus, these two experimental features are not well explained by the conventional
nonlinear resonant three-wave coupling theory. In the next section, we will describe our attempt
to overcome this contradiction by the help of computer simulations.

5. Computer Simulation of Nonlinear Interaction of Microwave
Power Beam with Space Plasma

Traditionally , methods of scientific research have involved a mutual interplay between experimentand theory. Experiment attempts to collect "factual " information through repeated or 169
controlled"Space and Radio Science" - 75th Anniversary of URSI
measurements. Theory, on the other hand, tries to order accumulated factual knowledge andthereby propose a new paradigm of description of physical processes. New theoretical descriptions

and newexperiments challenge each other in turn. Such feedback between theory and experimentis generally on-going. However,as in the case ofMINIX, theory and experiment sometimes show
a gap which cannot be easily overcome because of difficulties
of repeated experiments and/or alimit to the applicabilityof theory for highly nonlinear and complex processes. In order to fill sucha gap, a third new approach become available with the advent of modernhigh speed computers.
This third approach is called computer simulation or computer experiment. The basic idea ofcomputer simulation is to simulate the physical behavior of complicated natural systems bysolving an appropriate set of mathematical equations based on an accepted and fundamental
physical mathematical model. As one can easily change the set of the mathematical equations aswell as the boundary and initial conditions, the computer simulation can be a perfectly controlled
experiment. The main advantage of computer simulations is that complicated physical systems
including nonlinearity and/or strong inhomogeneity can be dealt with as easily as simpler, linear
and homogeneoussystems can be treated. The International Union of Radio Science (URSI) has
played a significant role of accelerating the establishment of this third research tool in Radio
Science, especiallyin the field space plasma wave studies through its activities
inCommission H [43].
In order to understand the MINIX result and the nonlinear interaction of the intense microwave
power beamwith the ionospheric plasma (which contradicts the nonlinear resonant three wave
coupling theory) weperformed a series of computer simulations. The computer code used for thispurpose is a particle-model simulation code called KEMPO[43]. KEMPO solves Maxwell'sequations and simultaneous equations of motion of several tens of thousands to several millionsof super-particles. The one-dimensional version of the KEMPO code is now available in the
public domain [42]. As the MINIX result showed that there exist two different electrostatic (ES)plasma waves with two different propagation angles relative to the geomagnetic field, we have
set up two different simulation models. One is the case where all ofthe incident intense microwave(or the "pump"electromagnetic (EM) wave in terms ofnonlinear resonant three-wave coupling),
the back-scattered microwave (or the back-scattered "idler" EMwave) and the excited ES plasma
waveare assumed to propagate along the external magnetic field Bo. The other model assumes
that they propagate in a perpendicular direction to Bo.The former and the latter cases are referred
to as the "parallel case" and the "perpendicular case" hereafter. The energy and momentum
conservation relation expressed by Eq.(2) can be graphically shown by a parallelogram in the
ohk diagram. Figure 13 is one example of the parallelogram in the perpendicular case. The
commonsimulation parameters for both cases are listed in Table 1. Figure 14 shows an example
of a temporal variation of the excited ES waveintensity, and that ofthe kinetic energy ofelectrons
for both the parallel and perpendicular cases. The upper and lower panels correspond to theparallel and perpendicular cases, respectively. In the parallel case, an L-modeEMwaveis adopted
as the pump. The L-modewavecouples with a back-scattered L-modeEMwaveand a Langmuir
wave(LW). This coupling is referred to as L-L-LWcoupling. A similar result is obtained (notshown) for R-R-LWwavecoupling with the R-modepumpwave.In the perpendicular case, an
X-modeEMwaveis used as the pump. The X-modepumpwaveis scattered producing a backscattered X-modeEMand the ES ECH wavethrough the X-X-ECHcoupling. A similar result isobtained for the O-O-ECHcoupling (not shown). The ECH waveexcited by the X-modeEM
1 70"Space and Radio Science" - 75th Anniversary of URSI

Fig. 13 -A parallelogram in the co-kdiagram
showing the resonant interaction between
twoelectromagnetic waves(microwaves) andelectron cyclotron harmonic (ECU) wave.

O-modewq: Pump Wave
wr. Idler Wave
We:Excited
Wave

wave,shown in the lower panel, is one of
the multiple harmonic modes of ECH
waves. It is the upper hybrid mode thatshrinks to the upper hybrid oscillation when
its wave number tends to zero. As seen in
Figure 14, both the LW and ECHWgrow
exponentiallyin the early phase. The
numerically measured growth rates of the
LW and ECHW are yLW=0.07 andYiW= 0.04, respectively.
These growth rates

agree well with the theoretically predictedvalue of the growth rate based on the nonlinear resonant three-wave coupling theory.
In contrast to a good agreement of the growth rates of both LW and ECHW,the saturation levelsof the LWand of the ECHWcontradict the theoretical prediction as seen in Figure 14. The LW
saturation level is almost half of that of the ECHW.The simulation result does not agree withtheory, but agrees well with the result of the MINIX rocket experiment. Thus, the computer
simulation could reproduce the inconsistency between the rocket experiment and theory. One of
the merits of computer simulations is that one can makethe diagnostics as detailed as one wishes
from the information storedin memory.In particular, compared with rocket experiments, one can<System Parameters><Pl asm a Pararneters>

S peed of L igh t
E lec tron P lasm a
A n gu lar F req uen cy
E le ctron C y clotron
A n g ular F req uen cy
E xternal M a gnetic
F ie ld S tren gth
C h arg e to M ass R atioo f E lectro ns
T otal N um b er of
E lectro n s
P arallel T herm al S peedo f E lectron s
Pe rp end icu lar T h erm al
S peed o f E lectro ns
D ie lectric C o nstant'SpaceandRadioScience"

c
n牀

50

T im e S te p

At

0 .0 1

2 .0

G rid S p a c in g

Ax

1 .0

N u m b e r o f G r id s

N r

2 0 48Qe

1 .0

B ex t1.0<Parameters of Injected EM Wave>q efm e‑ 1.0

Nr

32 76 8

VM

1 .0

vtkｱ

1 .0

｣o

.0

- 75th Anniversaryof URS1

A n gu lar F requ enc ycm

18.5 ‑2 3.4

W av e N u m ber(M od e N u m be r)
W av e M ag netic
F ie ld S tren gth

kn

15 ‑ 19

B (ｻ

0 .5

Table 1 - Numerical parametersfor
simulations presented in Figure 14-

171

Langmuir

Wave (7 = 0.07)(L mode Pump EM Wave)

~in-2
A>±\J

300

Electron

Cyclotron

Harmonic Wave(7 = 0.04) (X mode Pump EM Wave)

XlO"'

03

R ,

1.0H

0.0-

300

Fig. 14 - The left column shows time evolution of the electricfieldof electrostatic (ES) plasma waves
excited nonlinearly by the pump electromagnetic (EM) waves.The right column shows the correspondingtime history ofparticle thermal energy. The upper and lowerpanels arefor theparallel andperpendicularcases, Langmuir Waveis excited by an L-modeEMpumpwavewhile ECHWaveis excited by an X-mode
EMpumpwavein the perpendicular case.
extract detailed information on particle dynamics which is normally very difficult to measure witha sufficient time resolution. On the right panels in Figure 14, plotted are the time history of theparticle kinetic energy in the simulations.
In both the Langmuir wave and ECHW cases, theparticle thermal energy increases exponentiallyaccording as the electric field E grows exponentially,
until the electric field reaches the saturation. This initial exponential increase ofthe kinetic energy,
however, does not actually cause the thermalization
ofelectrons, but reflects the sloshing motion
synchronizing
with the electric oscillationof the ES wave. However, at the time of the wave
saturation, those particles begin to be thermalized through phase-mixing. The rise in the particle
thermal energy and the fall in the electric field intensity after the saturation show that the particles
start to extract energy from the ES wave and are heated.
Examination of the thermal energy increase helps us to understand why the saturation levels of
the electric field of the ES waves are much lower than those estimated by theory, and why themeasured Langmuir wave intensity is lower than that of the ECHW. To confirm the result of thefirst single simulation on the growth rate and saturation levels, we ran a series of simulations withdifferent frequencies of the pump EMwave.The simulation parameters for the simulation series
are listed in Table 2. The result is shown in Figure 1 5. The solid and dashed lines show theoreticalvalues of the LW and ECHW, respectively.
Those of the LW and ECHW observed in thesimulations are plotted by square and circular symbols, respectively.
The agreement ofthe growth
rates (Figure 15(a)) between theory and simulations were confirmed by this series of simulations.
172'Space and Radio Science"- 75th Anniversaryof URSI

<Plasma Parameters>

S p eed of L ig ht
E lectro n P lasm a
A ng ular F requ ency
E lectro n C yclo tro n
A ng ular F requ ency
E xtern al M agnetic
F ield S tren gth
C h arge to M ass R atioo f E lectro ns
T otal N um b er o f
E lectro ns
P arallel T herm al S peedo f E lectro ns
P erpen d icu lar T h erm al
S p eed o f E lectron s
D ielectric C on stant

(A)
a,

c

50

rie

2 .0

Qe

<System Parameters>
T im e S tep

At

0 .0 1

G rid S p acing

Ax

1.0

N um b er o f G rid s

N *2 04 81.0< P aram eters of Injected E M W av e>
A ngu la r F req uen cy(00 5 .0 ... 22 .5

B ex t1.0

qe/m e‑1.0

Nr

327 6 8vth¥¥

1.0 IO "4% tl

1.0 10 "4｣o

1.0

W av e N um be r
(M od e N u m b er)
W ave M ag netic
F ie ld S tren gthko

3 3 ... 109

B m

0 .5

Table 2 à"Numerical parametersfor
series of simulations presented
in Figure 15a(B)..o
s
fl t

4^ ‑

4
2
0
10

20

wo/fteTheoryLangmuir
Wave-- Electron Cyclotron Harmonic Wave
Computer Simulation
13 Langmuir Wave
© Electron Cyclotron Harmonic Wave

Fig. 15 - Comparison ofthe growth rates ofthe ES wavesand their saturation levels between theory(indicatedby lines) and computer simulations (indicated by symbols). The solid line and squaresymbols are for Langmuir Wave.The dashed line and circular symbols are for the ECHW.

Onthe other hand, the saturation levels observed in the series of simulations are muchlower than
the theoretical prediction shownin Figure 1 5(b). The largergap between the theory and simulation
forthe Langmuir wavecompared to the ECH waveresults in the reversal of the saturation levels
thereby contradicting the theoretical prediction. The saturation at lower intensity or at earlier timeof interaction for the Langmuir wavesturns out to be the result of the breakdown of the resonant"Space and Radio Science" - 75th Anniversaryof URSI173

condition (Eq.(2)). As seen in Figure 14, the plasma is heated by the nonlinearly growing ES wave
leading to heating of the plasma. Therefore, the fact that the earlier saturation than predicted by
the three-wave coupling theory should be related to the plasma heating. We then examined thechange of the di spersion characteristics
as a function of the electron temperature. The result showsthat the dispersion relation of the Langmuir waves has higher susceptibility
to the change of theelectron temperature. Figure 1 6 shows the change of the dispersion relation in the (o-k diagrams
when the electron temperature is doubled. As indicated in the figure, the increment of thefrequency Aco, which represents the frequency mismatching, is larger for the LW than for the
ECHW.The frequency mismatching given by A(0 = coq-cOi-g^influences tne growth of theexcited ES wave through [44],y =<J\plP2\E$
Electron

Cyclotron

3 .4 3(6)- Aa)

Harmonic Wave Langmtur Wave

221eo

6 .0 027 L

L ̲ I.
u¥2 .2 8¥4 .0 0
Tf
eO

o .

^
1 .14

ECHW

‑ r

2 .0 0TeQ

A ｫ LW0 .0 0
0.0

I
1.0

i

I
2.0

I

I

I

3.Qxw~*

0 .0 0

0.0

I

I

I

1.0

k/ <£le

I

I

2.0

t

i

l

3.0*lr

fcc / Oex Three wavecoupling point at initialcondition

Fig. J6 - Comparisonofthe change ofthe dispersion characteristics
ofLW and ECHWdue todoubling ofthe electron temperature. The quantity Dwgives a measure ofthefrequency
mismatching from the resonant condition of the nonlinear three-wave coupling.

Therefore, the change of the dispersion relation, or the breakdown of the resonance condition forthree-wave coupling due to plasma heating explains why the Langmuir waves reached saturation
earlier and thereby stayed at a lower intensity than ECHW.
Another discrepancy concerning the nonlinear excitation of plasma waves by the microwave
powerbeamrevealed by the MINIX rocket experiment was the broad band nature of the excited
Langmuir waves.The excited Langmuir wavesdid not show the line spectrum as predicted by thetheory (Eq. (2)), but showed a broad spectrum. This can be understood in the light of the resultsof the computer simulation. The previous example of the simulation shows an effective plasma
heating by the excited ES waves which self-quenches the wave-amplitude of the ES waves. This
174

'Space and Radio Science" - 75th Anniversaryof URSI

Pump EM Wave

Excited

ES Wave

U>2

Excited

ES Wave

1stThree W»veCoupling--2ad Three WaveCoupling

Fig, 1 7 - A schematic illustration
ofthe change ofresonance condition of the three-wave coupling
involving Langmuir Waves. Due to the change ofelectron temperature, anewtriplet(
6)0, C0l ', (02 ')is found automatically,
thus leading to thefrequency broadening.

Fig, 1 8 - A schematic illustration ofsimultaneous
nonlinear three-wave coupling by one EMpump
wavefeeding energy and momentum
into multiple
ECH waves.

meansthat the frequency and wavenumber of the triplet (c% o>| , fi^) and (k0, kl5 k2) changes intime so that a new triplet automatically satisfies Eq. (2) for the heated plasma. A schematic
illustrationof this interpretation is given in Figure 17. Such sequential shift of the resonancefrequency can explain the broad band nature of the observed Langmuir wavesin the MINIX.
Concerning the ECHW,another type ofnonlinear three-wave coupling is possible for the electron
cyclotron harmonic waves, as illustrated
in Figure 18. In this case, multiple triplets (<% coj, ct^),((% ct^, G)4), (ooq, q)5, o6) can satisfy the energy and momentumconservation simultaneously asa result of multiple branches of the dispersion relations in the G>k diagram. This feature can
explain the MINIX multiple spectral peaks of the ECHWwith a spacing of the order of the local
electron cyclotron frequency.
As discussed above, even a simple one-dimensional computer simulation based on the particle
model of the plasma turns out to be sufficiently
effective to fill a gap between theory andexperiment. Moreover the simulation solves an apparent discrepancy between the simple
nonlinear theory and rocket experiment. Extending the simulation model from 1-D to 2-D, we areable to study the nonlinear interaction morerealistically.
Figure 19 is an example of such a 2-D
simulation. Snap shots are shown of the spatial intensities of the pump wave(left upper panel),ofthe excited ES plasma waves (right upper panel) and ofthe thermal velocity ofthe plasma. The
pumpEMwaveis radiated from the left boundary by an array of current sources placed on the left
boundary, thus reproducing the spatially inhomogeneous intensity distributionof the MINIX
microwave power beam.The ES waves shownin the upper right panel have already been dampedin the vicinity of the antenna where the plasma is heated effectively.
"Space and Radio Science" - 75th Anniversaryof URSI175

Fig. 19 - A set ofthe result of two-dimensional
computer experiment by KEMPOonthe nonlinear
effects caused by the injected intense EM wave.
The upper leftpanel shows the intensity contour ofthe electric field of the pump EM waveinjectedfrom the left boundary. The upper right panel
shows the intensity ofthe excited ES waves. The
lower left panel shows the spatial distributionofthermal speed of the plasma. It is interestingtonote that the plasma electrons are heated in aregion where the ES waves are intense.

The microwave power beam used for the future SPS has to pass through the magnetosphere andthe ionosphere. In addition to the Ohmic heating and large-scale thermal instabilities
discussed
in Section 3, nonlinear excitation of electrostatic
plasma waves is highly possible taking into
account the theory, computer simulation and the rocket experiment MINIX. Though the power
absorbed by these ES plasma waves and resultant plasma heating is very small, the impact ofthe
microwave power beam onto the ionospheric plasma is not negligible.
Nevertheless knowing theplasma wavecharacteristics
excited by the microwave power beamas well as the physical plasma
process involved in the excitation,
we should be able to avoid possible interference to the HF
communication network.

6. Applicationof Microwave Power Transmission to
Microwave-Driven Airplane

In the late 1980's, a program to develop a long endurance high altitude platform called SHARP(Stationary
High Altitude Relay Platform) was proposed in Canada [45]. The idea is to float an
unmanned light-weight
airplane for a long period, circling at an altitude of about 21 km for the
purpose of relaying radio communications signals over a wide area. To maintain the platform
floating for weeks or months, a fuel-less airplane powered by microwave energy transmitted fromthe ground wasproposed and experimented on [46]. On September 17, 1 987, a 1/8-scale prototype
SHARP flew on beamed microwave power for 20 minutes at an altitude of about 1 50 m. Figure 20
shows a photo of the prototype SHARP with a 4.5 m wingspan. The microwave beam wastransmitted
by a 4.5 m diameter parabolic antenna transmitting
10 kW microwave with a

176-"Space and Radio Science" - 75th Anniversaryof URSI»<tes«-Fig. 20 - A 1/8-scale

SHARP Airplane and a parabolic antenna which will beusedfor 1/4-scale SHARP experiment.

frequency of2.45 GHz. Twowater-cooled magnetrons each with 5 kW output power were used.
The parabolic antenna mechanically tracked the airplane which flew inside a 50 degree cone. The
power density at the airplane altitude was 400 W/m2. A dual polarization
rectenna with two
orthogonal linearly-polarized
dipole arrays was developed. The rectenna diodes used in the firstflight were Silicon Schottky diodes (HP2835).
Its power handling capability
was 1 W/element,
and its microwave-to-DC conversion efficiency
was about 70% [46]. The rectenna received
sufficient power to feed 150W to the electric motor of the 4.1 kg weight SHARP airplane [46].

"Space and Radio Science" - 75th Anniversaryof URSI

177

«*!*

Fig. 2 1 - A schematic illustrationofStratosphericRadio Relay System (SRRS). Five SRRS's will
cover most ofthe heavy communication demands
in Japan.

Asimilarproject
was carried out in Japan in the early 1990's. The project was called StratosphericRadio Relay Systems (SRRS), and was studied by a working group under the Ministry of Posts
and Telecommunications of Japanese government [47]. The objectives of the SRRS are similar
to those of Canadian SHARP. In the SRRS, it is planned to launch five such unmanned airplanes
over Japan as depicted in Figure 21, so that these five platforms can cover most of areas where
communication demands are heavy. In parallel with the SRRS working group, a microwavedriven airplane experiment wasplanned and conducted successfully on August 29, 1992 by ajoint
team organized by the present author [48]. The team members were from Kyoto University, Kobe
University, Communications Research Laboratory, Nissan Motor Co. Ltd., Fuji Heavy Industries
Ltd. and Toshiba Co. The experimental project was called MILAX meaning Microwave Lifted
Airplane experiment, and was partly sponsored by ISAS of Japan. The MILAX airplane is abalsa-based light-weight(~ 4 kg) airplane with a 2.5 m wingspan and has a shape as shown in
Figure 22. The MILAX flew successfully for40 seconds (or 400 m distance over a straight course
forcardrivingtest) at an altitude of about 15 m. The testing scenery is shown in Figure 23. Because
ofthe limits ofthe maximummicrowave power (~ 1 kW) and ofthe aperture ofthe transmitting
antenna (~ 1.2 m) , the flight altitude had to be as low as 15 m in order to guarantee the power
density of 200 W/m 2at that altitude. The microwave power beam was radiated toward the fuelfree MILAX airplane by an active phased array antenna. The MILAX active phase arraytransmitter was composed of five-stage Gallium-Arsenic(GaAs) semi-conductor amplifiers (seeFigure 24), 4-bit digitalphase shifters and circular microstrip antennas (see Figure 25). The
transmitter is divided into 96 sub-arrays, each consisting of3 antennas, one phase-shifter and one

178"Space and Radio Science" - 75th Anniversaryof URSI6Z. I

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Fig. 24 - GaAs-based semiconductor amplifiers used in the MILAX.
Each amplifier supplies 13 W microwave output.

Fig. 25 - A view ofthe transmitting antenna array installed on the roofofa transmitter car.
The antennas wereof circular microstrip type.
GaAs amplifier. Each sub-array can supply 1 3 W microwave output resulting in the total radiation
capabilityof 1.25 kW. The frequency used in the MILAX was 2.41 1 GHz in the ISM frequency
band. The transmitter system was installed on the roof of a transmitter car (see Figure 25).
Six rectenna subarrays, each consistingof 20 rectennas are installedon the flat-bottom of the
MILAX airplane. Prior to the development of the MILAX rectenna, several rectenna researches
had been done in Japan [49, 50, 5 1]. Based on these studies, the receiving antennas used for the
MILAX rectenna were not of the dipole-type,
like these used in the JPL/Goldstone
Ground-toGround Power Transmission Experiment and in the MINIX and SHARP, but were ofa new type180

"Space and Radio Science" - 75th Anniversaryof URSIIllllllllllllllllli

Honeycomb

Antenna Element/

Pat chSlot/

Kapton Film

Ground Plane
Teflon* Rectifying
CircuitFig. 26 - A microstrip circular antenna-based
reclenna usedfor the MILAX project [50].

Output Filterof microstrip circular patch antennas. The circular patch antennas have the advantage of a nonresonant nature at integer multiple harmonic frequencies, thereby having the capabilityofsuppressing
spurious radiationfrom the rectennas. The disadvantageof heavier weight ascompared to dipole antennas was overcome by introducing a paper honeycomb structure [52], asshown in Figure 26. The diodes used for the MILAX rectenna are eight HP5082-2350 Schottky
diodes in 2-series /4-parallel
combination. The power handling capability
was 1 W perelement,
and the microwave-to-DC conversion efficiency was about 52 % [52].
The main reason of the adoption of the active phased array in place of a conventional parabolic
antenna is its higher steerability
ofthe microwave powerbeam. The powerbeam can be controlled
and steered electronicallyin contrast to the mechanical control of a parabolic antenna. In the
MILAX, we monitored the location of the MILAX airplane by two CCD cameras which were
installedon the edge of the roof-transmitter
antenna looking upward. In Figure 27, a system ofidentifying
the location ofthe airplane is shown. A micro-computer, after recognizing the pattern

Phas.'
S hift,r: Miiruwavi; Trausmll(.«åFig. 27 - A computer-controlled beam steering system for microwave power transmission toward the
MILAXairplane. The image ofthe airplane is captured by two CCD cameras installed on the roofofthetransmitter car. Then the computer recognizes the airplane location and height by a pattern recognition
software. According to the information on height and location, the computer controls the phase shifters
ofthe microwave amplifiers of the active phased array.
'Space and Radio Science"- 75th Anniversaryof URSI181

of the airplane image and calculating the x-y coordinates and the altitude of the airplane, sends
the control signals to the phase shifters of the microwave amplifiers so that the microwave beamis accurately directed toward the airplane. This system worked perfectly in the MILAX.
Though weadopted computer-control for steering the microwave power beam in the MILAX due
to time and budget limitations,
we have also attempted to develop the retro-directive
beam control
system. Figure 28 shows a photograph ofa 90 W microwave power transmitter with the use ofthe
retro-directive
control method [53]. Seven dipole antennas are connected to semi-conductor
microwave amplifiers. The 90 Wmicrowave power transmitter has capabilityof transmitting
themicrowave power beam automatically in the direction of incoming pilot signals. A unique featureofthe developed unit shown in Figure 28 is that it uses a newphase conjugate circuit (PCC) using
asymmetric two pilot frequencies. The PCC used for the unit is shown in Figure 29. As we use
two asymmetric pilot signals (Oq + Acoand (Oq+ 2Aooinstead of (Oq± A(O,we need no division of
the phase 0, of the pilot signal after the first mixer. Therefore we can determine the phase 0jt
uniquely without the ambiguity of n radians.

Fig. 28 - A 90 Wmicrowave power transmitter unit composed of seven dipole antennas, seven GaAs
semiconductor amplifiers and retrodirective phase conjugate circuits.

182"Space and Radio Science" - 75th Anniversaryof URSI

<t>t + 2A0u)t + 2Au;<t>t®-e- 2ut
Fig. 29 - A new phase conjugate circuitasymmetricfrequencies

usedfor the 90 W unit in Figure 28. Twopilotof (00 + Aft) and ft)0 + 2Aft) are used.

signalswith

7. Other Recent Experiments on Microwave Power Transmission in JapanOn Feb. 1 8, 1 993, a second rocket experiment of microwave power transmission between motherand daughter units was carried out by the S-520-16 sounding rocket [54, 55]. The rocket
experiment was given the name of ISY-METS meaning Microwave Energy Transmission inSpace during the International
Space Year. The ISY-METS had objectives of investigating
nonlinear effects of the high power microwave onto the ionospheric plasma in more or less asimilar way to that of the MINIX. In this sense, the ISY-METS is an advanced version ofthe
MINIX. However, it has another mission to verify a newly developed active phased arraymicrowave transmitter which had been modified from that used in the MILAX in the space plasma
environment. The total power of approximately 800 W wastransmitted from the microstrip array
Daughter Section
VLF-Airtmna

HF-B

HF-Antmna

TM-Antwtna
HF-A

RectMita

Mother Section
Fig. 30 - Picture and illustrationof the payload
section of the 1SY-METS rocket experiment. Four
deployed paddles mount 16 transmitting microstrip
antennas each. The daughter unit carried diagnostic
packages and rectennas with various sensors.
"Space and Radio Science" - 75th Anniversaryof URSI

183

antennas mounted on four deployed paddles. The configuration of the transmitting antenna
paddles and the sensors extended outward from the daughter unit are shown in Figure 30. The
phase of the transmitted microwave from each antenna on the deployed paddle wascontrolled by
the same4-bit digital phase shifter as used in the MILAX. The phase shifters were controlled byan onboard computer providing a variable transmitted power density and direction of the
microwavebeam.Figure 3 1 shows an example of power concentration at apointof approximately
4.5 mawayfromthe center of the transmitting antennas. Such power-concentrated points were
determined by the computer using pre-set parameters and the onboard real-time data of the relative
direction the daughter unit. Figure 32 shows a data set during a pre-flight test in the radio anechoic
chamber. The left panel shows the antenna pattern of the microwave transmitter. The right panel
shows the measured power at concentrated points which were varied from 1 m to 10 m from thetransmitting antenna. The measured data are shown by the solid line in the figure. The dots in thesamefigure are the power density measured onboard the IS Y-METSrocket during the flight; thesemeasurementsand agreed well with the pre-flight test values [55].
3 .0 4W

M M:

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‑3.0

m

nax-z(m) = 0 . 2

p max(V/m) - 1 1 7 3^ ‑ r,-*<ro)£(V/m)

X-axis(m)Fig, 31 - Computedpower
concentration m a pin the x-zandx - yplane, wherethe contour ofthe ISYMETStransmitting antenna isplaced atx = 0, y = 0 andz - 0 in the x-yplane. In the specificexample,t he power-concentration
point is set at (x, y, z) = (0 m, 2 m , 4 m)
̲ 20
E
CD
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Id
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15 .

5

10

DISTANCE |m]Fig. 32 - Antenna pattern ofthe ISY-METS microwavepower transmitter (left), and the measured
power (right) atpower-concentratedpoints
at distancesfrom O m to 10 m. The solid line and dots
represent the values measured in the pre-flight radio anechoic chamber,and in the flight in space, respectively.
184"Space and Radio Science" - 75th Anniversary

ofURSI

Fig. 33 - A photo showing afield experiment of MPT at Yamazaki Experimental
Site ofKansai Electric Power CompanyInc(KEPCI).
The experimentis a joint
collaborative
research between Kyoto
University, Kobe University and KEPCI(K3 project).

Twotypes of rectennas were installed
onthe daughter unit. One wasdevelopedby a research group at Texas A&M
University, while the other was by the
CRL group. The former used anorthogonally placed pair of three dipole
antennas, as used in the MILAX. The
structure and configuration are found in
Figure 9 and Figure 10 in [55]. The
initial results of the ISY-METS rocket
experiment are nowunder analyses andwill be published in detail elsewhere.
Other applications of microwave powertransmission
have recently gatheredinterests in Japan in the practical worldin addition
to the academia. One ofthem is a small-scale ground-to-groundpower transmission without wires toward a distant place where wired power distribution
networks
are either unavailable
not or very poorly available.
In order to collect fundamental data on
microwave power transmission under varying weather conditions, the Kansai Electric Power
Company Inc. began a collaborative field experiment with Kyoto University and Kobe University.
Figure 33 shows aphoto ofthe experimental site showing a parabolic antenna with a 3 m diameter
driven by a 5 kW magnetron and arectenna array ofa size of3.5 m x 3.2 mplaced42 mawayfrom
the parabolic transmitting
antenna. The rectenna array was built by Kyoto University and iscomposed of2304 rectennaelements. A preliminary test started in October, 1994 and is still being
conducted to collect fundamental data on the characteristics of power transmission and reception
by the system.

"Space and Radio Science" - 75th Anniversaryof URSI

185

8. Discussion

and Conclusion

The Tesla idea of wireless power transmission was revived by the NASA/DOE SPS studies
program in the 1 970's. Following the pioneering work on microwavepowertransmission by W.
C. Brown,manyengineers and scientists have conducted the related research and developed
applications stimulated by the SPS studies program in
the US, former Soviet Union, France, Germany, Japan and other countries. However, even after
the SPS boom subsided following the suspension of the SPS research in the US in 1981, both
fundamental academic research and application-oriented
developments and experiments in
microwavepowertransmission have continued. Radio scientists have been a core of such researchand development because the power transmission by radio inevitably involves problems in radio
science such as those on antennas, rectennas, propagation characteristics and nonlinear interactionof microwave
power beam with plasmas and neutral environments. In this paper, we have emphasized thedevelopment of such studies after 1 980 as studies before that time was well documented (e.g.[ 1]).
Since 1980, many feasibility
studies have been made and a variety of newideas of utilization of
microwavepowertransmission have been proposed. However,as far as the present author knows,
very few experiments have actually been carried out except for those discussed in the present
paper except for a laboratory-based development and research of rectennas.
There are manyplans and proposals for MPT applications, e.g., a French plan of a medium-scale
wireless power transmission (~ 100 kW, 3 km) on Reunion island, a proposal named WISPER(Wireless Space PowerExperiment) in the US which aims at ground-to-space power transmissionof the order of 100 kW, and a test project called ALASKA 21 which plans to transmit electric
power via microwave to scattered villages in Alaska. As well as these, private industries have theirownMPTprojects, while national research institutes will continue fundamental researches.
In Japan, MITI (Ministry of Trade and Industry) has re-examined the SPS feasibility
based onrecently developed technologies after 1980. Japanese research groups at ISAS and national
universities
are accordingly pursuing a conceptual study on an orbiting lOMW-scale Powersatellite called SPS2000 which beams downthe electricity to equatorial countries [56] (Figure 34)
and onapower satellite called PSS (Power Supplying Satellite) which feeds electricity ofthe orderof 100 kW to other orbiting customers [57] (Figure 35).
In summary,microwavepower transmission has been oneofthe interesting topics in radio science
both technologically and scientifically
However,the use of radio wavesas a meansof transmitting
electric power and energy is still in a very immature phase. More research and development will
be needed before the dreams of Hertz and Tesla became reality. Radio scientists and engineers
have great challenges to face in this new field.

1 86

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Acknowledgments
I would like to express my thanks to N, Kaya ofKobe University for his collaboration on this topic
for manyyears. Without his contribution most ofthe experimental projects described in this paper
would not have been successful. I also thank N. Shinohara, T. Miura and K. Miwa for their help
in preparing this manuscript, and D. Summersfor his careful reading of the manuscript.

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and J. Kochiyama,

A feasibility

study of Power Supplying----___^^_^"Spaceand Radio Science" - 75th Anniversary of URSI